Annular heat pipe

ABSTRACT

Spaced inner and outer tubes form a closed annular chamber whose inner surfaces contain coverings of wick material that are interconnected through vane-like wicks. The wick material transports a vaporizable working fluid from cold areas where the vapor condenses to warm areas where the fluid vaporizes. An isothermal working space is produced within the central volume bounded by the inner tube and along its entire length, which may be used to advantage for oven or furnace applications or for providing an isothermal jacket.

United States Patent Kirkpatrick July 18, 1972 ANNULAR HEAT PIPE1,987,119 1/1935 Long ..219/325 2,820,134 1/1958 Kobayashi........165/l05 X [72] Invent 2 '5" g' verdes 3,327,772 6/1967 Kodaira..165/1o5 x I 3,229,759 1/1966 Grover... ..l65/105 [73] Assngnee: TRWInc., Redondo Beach, Calif. I OTHER PUBLICATIONS [22] Filed: 1970Deverall, J. Efet 211., High Thermal Conductances Devices, [21] Appl.No.: 89,705 Los Alamos Scientific Lab., Univ. of Cal., 4/1965, pp. 1,34,

35. Related US. Application Data Pn'mary Examiner-Albert W. Davis, Jr.)1v1s1on of Ser. No. 797,725, Jan. 31, 1969, Contmua- Attorney Danie| T.Andemn, Jerry A Dimudo am Donald tlon-in-part of Ser. No. 637,193, May9, 1967, aban- R. Nyhagen doned.

[52] US. Cl. ..l/l05, 219/326, 219/399 [57] ABSTRACT [51] Int. F281115/00 Spaced inner and outer tubes form a closed annular chamber 58Field 6: Search ..165/l05; 219/325, 326, 399, whose inner surfacescontain coverings 9f wick material that 219/400 406 are interconnectedthrough vane-like wicks. The wick material transports a vaporizableworking fluid from cold areas [56] References cm where the vaporcondenses to warm areas where the fluid vapon'zes. An isothermal workingspace is produced within UNITED STATES PATENTS the central volumebounded by the inner tube and along its entire length, which may be usedto advantage for oven or fur- 2,616,628 1 1/1952 Guild 165/105 x aceapplications for providing an isothermal jacket 3,405,299 10/1968 Hallet aL... 3,490,718 l/l970 Vary ..165/ X 7 Claim, 9 Drawing FiguresPmmanauuemz 3.677.329

SHEE 1 OF 4 Milton E. Kirkpatrick INVENTOR.

AGENT PATENTEDJULIBIQIZ 3.671.329

SHEET 2 0F 4 lnslde Hoe! Pipe 7 2-0 sec Outside Heo'Pipe Temperature, C

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0 2 4 6 B IO l2 Distance, Inches Milton E. Kirkpatrick INVENTOR.

AGENT Pmmmmwmz 3,677,329,

SHEET 3 0F 4 a. gM

AGENT Fig.7

PATENTED JUL 1 8 m2 SHEET 0F 4 Milton E Kirkpatrick INVENTOR.

BY M G.

AGENT ANNULAR HEAT PIPE CROSS-REFERENCE TO RELATED APPLICATION Thisapplication is a division of application Ser. No. 797,725, filed Jan.31, 1969, which in turn is a continuationin-part of application Ser. No.637,193, filed May 9, 1967, now abandoned.

BACKGROUND OF THE INVENTION I 1. Field of the Invention This inventionrelates to heat transfer devices, and particularly to devices employingcapillary fluid transport, which are of such configuration that lendsitself especially to oven or furnace applications.

2. Description of the Prior Art The concept and art of building refluxboilers is well developed and dates back to papers on the subject duringthe 1930's. A heat pipe works on the principle of a reflux boiler and isextremely efficient in terms of transferring large thermal heat fluxes.Example of heat pipe devices are described in US. Pat. Nos. 3,152,774and 3,229,759, issued to T. Wyatt and G. M. Grover, respectively. Thebasic heat pipe is a closed tube which has a layer of porous wickmaterial attached to the interior surface of the tube wall. The tube orpipe is partially filled with a fluid, the specific fluid beingdetermined by the temperature range desired, which wets the porous wickmaterial and spreads throughout the wick material by capillary forces.

When a sufficient heat flux is applied to any point on the surface ofthe pipe, liquid will be vaporized. Energy equivalent to the heat ofvaporization is carried away from the high heat flux region by the vaporthat migrates throughout the interior regions of the pipe. The vaporwill recondense on any and all interior surfaces which are attemperatures below that of the vaporizing surface, thereby giving up theheat of vaporization to all cooler surfaces. t

The recondensed fluid is then transported by capillary forces back tothe vaporization region, or high heat flux input zone, to continue theclosed loop process of transporting and delivering thermal energy to anyand all cool regions of the pipe. As a result of this action, the heatpipe," although heated only in one small region, quickly becomes anisothermal surface; i.e. all surface temperatures on the pipe are equalor nearly equal no matter what the distribution of heat flux input maybe.

Inasmuch as the present invention may advantageously be used to providea diffusion furnace for the semiconductor industry, a brief descriptionof such diffusion furnaces will be given. In the present state of theart, a diffusion furnace has a long processing tube two to three feet inlength and several inches in diameter. The processing tube is surroundedalong its length by a long helical heating coil, which is divided intothree coil portions, namely a long central portion and two shorter endportions. The three coil portions are separately supplied withelectrical heater power so as to produce three separatelythermostatically controllable heating zones within the processing tube.The three zones are necessary to achieve a flat temperature profilealong the longest possible length of the processing tube. A flattemperature profile is necessary to assure that all the semiconductorwafers, which are placed in a boat within the processing tube, will besubjected to the same thermal diffusion processing conditions. Thediffusion process consists of introducing a gaseous impurity or dopantmaterial into the processing tube while the boatload of semiconductorwafers are heated at about 1300 C.

Despite such an elaborate three zone heating arrangement, substantiallyless than the entire length of the processing tube attains a flattemperature profile. Furthermore, some rather complex electrical controlcircuitry is required to control or modify the temperatures of the threezones so that the furnace not only attains a flat temperature profilebut also maintains it under different boatload conditions.

SUMMARY OF THE INVENTION The present invention resides in a uniquelyconfigured structure utilizing the basic "heat pipe" concept, and in therecognition that such structures advantageously can be used and ought tobe used in certain types of furnaces, such as diffusion furnaces.Interior surfaces of an annular pipe are provided with a porous wickmaterial, such as sintered metals, wire screens, or other porouscompacts, to provide complete interconnection of fluid flow paths. Whencharged with a working fluid which wets the wick material and has asuitable vapor pressure matching the desired temperature range ofinterest, the isothermal annular heat pipe may be used for a variety ofheat transfer applications. In particular, when employed in a diffusionfurnace, it will produce a flat temperature profile along the entirelength of the processing tube or chamber by the use of a singleelectrical heater coil and a single temperature controller.

BRIEF DESCRIPTION OF THE DRAWING FIG. 6 is a perspective view, withportions removed, of a diffusion furnace employing an annular heat pipeaccording to the invention;

FIG. 7 is a cross-sectional view of the furnace assembly of FIG. 6; and

FIGS. 8 and 9 are perspective views of alternative forms of diffusionfurnaces employing annular heat pipes of rectangular cross-section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there isshown an oven or furnace 10 provided with a central isothermal workingspace 12 formed within an annular heat pipe 14. An electrical heatercoil 16 is wound around one end of the heat pipe" 14 and receiveselectrical power from a voltage source 18. The heater coil 16 may beembedded in a thermal insulation sheath 20 that surrounds the heat pipe"14 along its length. The insulation sheath 20 serves to minimize heatloss from the fheat pipe" 14 to the surrounding atmosphere.

As shown more clearly in FIGS. 2 and 3, the heat pipe" 14 includesconcentric inner and outer cylindrical metal tubes 22 and 24respectively. The space between the tubes 22 and 24 forms an annularchamber 25. The surfaces of the tubes 22 and 24 disposed within theannular chamber 25 are covered with linings 26 and 28 of porous wickmaterial. The two wick linings 26 and 28 are spaced apart and joinedtogether by short spacer elements 30 of wick material that are spacedalong the length of the tubes 22 and 24.

The annular chamber 25 is closed at both ends by cover plates 32, suchas the one shown in FIG. 1, which leave the isothermal working space 12open for easy access from the outside. The annular chamber 25 isevacuated of non-condensable gases, such as air, and contains avaporizable fluid 34 of sufficient quantity to wet the entire wickmaterial by capillary action. The specific fluid depends upon theoperating temperature desired for the heat pipe" 14.

The wick material for the linings 26 and 28 and spacer elements 30 maybe in the form of sintered metal, wire screens, or other porous compactshaving voids or openings of capillary size and capable of transportingthe vaporizable fluid 34.

In the operation of the furnace 10, the heater coil 16 is energized toheat the portion of the annular heat pipe" 14 surrounded thereby. Thefluid 34 heated thereby vaporizes and the vapor carries away from thehigh heat flux region thermal energy equivalent to the heat ofvaporization. The vapor migrates through the annular chamber 25 where itcondenses on all interior surfaces that are below the temperature of thevaporizing surface, thereby giving up the heat of vaporization to andraising the temperature of all the cooler surfaces. Continuous vaporflow paths are provided along the annular extent of the annular chamber25 by means of the linear spacing between the spacer elements 30. Thecondensed fluid 34 is then transported by capillary action through thewick material from these condensing regions to the vaporizing region orhigh heat flux input zone, where the fluid 34 again vaporizes.

By means of this closed loop process, thermal energy supplied by theheater coil 16 is transported and delivered to any and all coolerinterior regions of the chamber 25. The result is that the entiresurface of the heat pipe" 14 quickly becomes an isothermal surface whenoperating in the temperature range specified by the working fluid, andthe volume within the isothermal working space 12 of the furnace 10 isuniform in temperature along the entire length of the heat pipe 14.

For a specific working fluid, there is a range of equilibriumtemperatures over which the device of this invention will provideisothermal conditions. The lower limit of the equilibrium temperaturerange is detennined by the thermodynamic properties of the workingfluid, namely the vapor pressure and the heat of vaporization. The upperlimit of the equilibrium temperature range is determined by themechanical ability of the device to withstand the positive pressures ofthe vapor relative to the surrounding atmosphere.

The working space 12, being devoid of fluid 34, can be used as an ovento process various articles of manufacture, such as semiconductivedevices, without danger of contamination by the working fluid 34. Inaddition, the furnace 10 may be used to provide an isothermalenvironment for various components requiring uniform thermaldistribution, with the oven shaped in conformity therewith. Accordingly,whereas the furnace 10 has been shown as having a circular, cylindricalshape, it may have a rectangular cross-section or even a complexcross-sectional shape.

In accordance with an exemplary operative embodiment, the tubes 22 and24 consisted of stainless steel cylinders having lengths of 12 inches,inside diameters of 1.5 inches and 1.9 inches respectively and outsidediameters of 1.6 inches and 2.0 inches respectively. The wick materialwas fabricated from four multiple layers of 100 mesh stainless steelscreen. The vaporizable fluid was potassium metal.

For applications where temperatures in excess of l000 C are required,such as in the treatment of semiconductor devices, working fluids suchas lithium or other liquid metals having the desired vaporizationtemperature can be employed.

For applications requiring high operating temperatures it will proveadvantageous to utilize silicon carbide rods as heating elements, ratherthan heater coils. In FIG. 4, for example, two or more such rods 35 maybe arranged side by side beneath the annular heat pipe 14 as shown. Therods 35 may be connected in parallel with the voltage source 18.

FIG. shows curves of temperatures taken along the length of the furnaceat two different regions thereof. Curve 36 refers to the regionindicated in FIG. 1 by dashed line 38 as occupying the space adjacent tothe inside surface of the insulation sheath 20. Curve 40 refers to theregion on the interior end. In contrast, the temperature on the interiorsurface of the inner tube 22 is uniformly at 720 C along the entirelength.

The present invention provides special advantages when used forprocessing semiconductor devices. For example, in the art ofsemiconductor device manufacture, it is necessary to heat wafers ofsilicon in the presence of dopant materials in a furnace or oven attemperatures of the order of i000 C. With furnaces in present use, thetemperature is fairly uniform in the central portion but dropssubstantially at the ends. As a result, about 60 percent of the furnacelength is unusable. With the present invention, substantially the entirelength of the furnace is uniform in temperature, and a much greaterlength of the furnace zone may be used for treating semiconductordevices. Consequently, in a particular application, the equivalent powerconsumption for the processing can be significantly reduced.

Furnaces presently in use utilize several electrical heaters distributedalong the furnace length, and each of the heater coils may beindividually thermostatically controlled. Maintenance problems arisefrom the fact that failure can occur from one of the number of heatingcoils and control circuits. Maintenance problems as well as systemscosts are reduced in the present invention in that only a single heatersource and control circuit are required.

The present invention provides additional special advantages when usedfor elevated temperature mechanical property testing. In the art ofproperty testing, a furnace is em ployed to heat the subject specimen.Furnaces in present use in the art, employ a multiplicity of heatercoils along the length of the furnace which are individually controlledand adjusted to provide semi-uniform temperature over the active region.When adjusted, such a furnace will provide temperature uniformity ofseveral degrees variance over the length of interest. During the testsequence, any change in heat balance due to changing test conditionswill effect and degrade the thermal uniformity within the furnacevolume. The present invention eliminates the need for any manual orsemiautomatic adjustment of the position of thermal input or temperatureuniformity within the volume of the isothermal working space. A singleautomatic temperature control is thus all that is required to maintainuniformity, throughout the working volume, over any desired temperaturewithin the working range of the heat transfer fluid.

The invention will now be described as applied to the con-. struction ofa diffusion furnace for processing semiconductors. Referring to FIGS. 6and 7, there is shown a diffusion furnace consisting of a furnaceassembly 50 and a power control system 52 The furnace assembly 50includes an outer cabinet or casing 54 lined with thermal insulation 56.Extending longitudinally and centrally of the casing 54 and supported bythe insulation 56 is a helical heating coil 58 that is wound around acylindrical ceramic support tube 60.

The heating coil 58 serves the same function as the heater coil 16 ofFIG. 1, namely that of supplying heating energy to the annular heat pipe20, which in this diffusion furnaceis supported within the support tube60. To this end, the heating coil 58 may be formed of high resistancewire that will heat to a high temperature when supplied with electricalcurrent of 60 cycle frequency. Alternatively, the heating coil 58 maycom-' prise metal tubing of high electrical conductivity which, when.furnished with radio frequency current, will cause the annular heat pipe20 to heat up by electromagnetic induction.

A cylindrical processing tube 62 made of suitable material such asquartz is supported within the annular heat pipe 20 and extendslongitudinally through both ends thereof. The processing tube 62 isprovided with an open end 64 through which may be inserted a boat 66containing wafers 68 of silicon or other semiconductor. The other end ofthe processing tube 62 is provided with a smaller opening 70 throughwhich a suitable gaseous dopant material may be introduced into theprocessing tube for diffusion into the semiconductor wafers 68.

It will be observed that the wafer-loaded boat 66, or a pluralitythereof arranged end to end, may extend substantially the entire lengthof the annular heat pipe 20 and even beyond the extremities of theheating coil 58. The reason for this is that the effective heating zonefor heating the semiconductor wafers 68 is determined by the interior ofthe annular heat pipe 20 rather than the heating coil 58. The effectiveheating zone has a flat temperature profile along the entire length ofthe annular heat pipe 20.

The power control system 52 includes a power supply 72 for furnishingelectrical energy to the heating coil 58. The power supply 72 isconnected to the heating coil 58 through a controller 74. A thermocouple76 contacting the annular heat pipe 20 is connected to the controller74. The thermocouple 76, which may be supported in a tube 77, as shownin FIG. 7, senses changes in heat pipe temperature above and below agiven set point for which circuits in the controller 74 are set. Thecircuits in the controller 74 operate to turn on power to the heatingcoil 58 when the temperature falls below the set point and to turn offpower to the heating coil 58 when the temperature rises above the setpoint.

Temperature control systems for diffusion furnaces are well known in theart and therefore the controller 74 requires no further detaileddescription. It will suffice to say that the controller 74 may be one ofthe kind disclosed in U. S. Pat. No. 3,291,969 issued Dec. 13, 1966, toB. J. Speransky et al, for controlling the central zone B of the heatingcoil 11 of that patent.

The power supply 72 may be designed to furnish 60 cycle alternatingcurrent to the heating coil 58 if the latter operates on the principleof resistance heating. On the other hand, if the heating coil 58 is anelectromagnetic induction heating coil, the power supply 72 may bedesigned to furnish radio frequency current to the heating coil 58.

It will be seen that the diffusion furnace thus described is muchsimpler in the construction of its furnace assembly 50 and its controlsystem 52 than the corresponding structure of conventional diffusionfurnaces. The inclusion of an annular heat pipe according to theinvention permits the use of a single heater coil instead of three and asingle temperature control system instead of three. The annular heatpipe 20 provides a flat temperature profile along its entire interiorlength, thereby increasing the capacity of the semiconductor processingzone. Furthermore, whenever it is desired to change the temperature ofthe furnace, the temperature will rise or fall uniformly along theentire length of the heating zone.

Referring now to FIG. 8, there is shown a modified form of diffusionfurnace assembly 50a which has a rectangular crosssection. Thus, theprocessing tube 62a and annular heat pipe 20a are rectangular instead ofcircular. A heater element 58a of flat sinuous form is mounted adjacentto a surface of the heat pipe 20a, such as the top surface thereof. Thewindings of the heater element 58a extend at an angle to thelongitudinal axis of the heat pipe 20a and processing tube 62a. Withthis flat configuration, it is preferable that the heater element 580 beof the resistance wire heating type. The heater element 58a may bedesigned for direct thermal contact with the annular heat pipe 20a. Forexample, the heater element 58a may comprise a central current carryingconductor 78 spaced from an outer metal sheath 80 by electricalinsulation 82. Altematively, for convenience in assembly or disassembly,the heater element 58a may be mounted on a flat support member 60a,which itself is mounted on the heat pipe 20a. For ease in illustration,the remaining parts of the furnace assembly 50a are not shown, it beingunderstood that it contains similar parts corresponding to theinsulation 56 and casing 54 of FIGS. 6

and 7. Likewise, a control system 52 similar to that already describedin connection with FIGS. 6 and 7 may be used with the rectangularfurnace assembly 50a.

An additional advantage of incorporating an annular heat pipe in adiffusion furnace is apparent in FIG. 8. That is, the heating element58a need not envelope the processing tube 62a, as is required inconventional diffusion furnaces. It is sufficient to apply all therequired thermal input energy to a localized area of the heat pipe 20a,such as the top surface or a portion thereof, and through the operationof the heat pipe 20a, the entire surface area thereof will attain anisothermal condition. Furthermore, it is not necessary, in the design ofthe heater element 58a, that great regard be given to precise spacingbetween turns or windings, or in uniformity in the lengths of thewindings.

FIG. 9 shows another form of rectangular diflusion furnace 50b that issimilar to that of FIG. 8. In this embodiment, the a heater element 58bhas sinuous windings that extend parallel to the longitudinal axis ofthe heat pipe 20b and process tube 62b. The heater element 58b, whichmay be mounted on a support tube 60b, may cover all four sides of theheat pipe 20b both longitudinally and circumferentially, as shown, or itmay cover a less number of sides or only portions thereof.

A principal advantage of a rectangular configuration for the diffusionfurnace assembly is that it minimizes the cross sectional area of theprocessing tube required for any boat and semiconductor loadconfiguration. This minimizes the heat loss from the open ends of thefurnace and improves the temperature profile thereof.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In combination:

a tubular member having spaced inner and outer walls and interconnectingend walls defining a closed, evacuated annular chamber surrounding acentral opening;

means forming a capillary structure on the inner surface of said outerwall and a capillary structure on the outer surface of said inner wall;

additional capillary means interconnecting said capillary structures andincluding a plurality of groups of wick elements spacing said capillarystructures, said wick elements being circumferentially andlongitudinally spaced within said annular chamber to provide continuousfluid flow paths through said chamber;

and said annular chamber being evacuated of all non-condensable gasesand being partially filled with a substance which is vaporizable from aliquid phase and is capable of transport through said capillarystructures and additional capillary means.

2. A heat pipe comprising:

a tubular member having spaced inner and outer walls and intersectingend walls forming a closed, evacuated chamber including an annular spacebetween said spaced walls;

a capillary structure on the inner surface of said outer wall and acapillary structure on the outer surface of said inner' wall;

additional capillary means interconnecting said capillary structures andincluding a plurality of groups of wick elements spacing said capillarystructures, said wick elements being circumferentially andlongitudinally spaced within said annular space to provide continuousfluid flows paths within said chamber; and

a working fluid partially filling said chamber and formed of a substancethat is vaporizable from a liquid phase and is capable of transportthrough said capillary structures and said additional capillary means.

3. A heat pipe comprising:

a pair of radially spaced inner and outer walls extending longitudinallyand circumferentially and closed by end walls to form a hermeticallysealed chamber including an annular space between said radially spacedwalls;

a capillary structure on the inner surface of said outer wall and on theouter surface of said inner wall;

members to form an elongated annular chamber surrounding an openpassageway. 5. The invention according to claim 4, wherein said tubularmembers are circularly cylindrical.

6. The invention according to claim 3, wherein said capillary structuresand said wick elements are formed of metal mesh.

7. The invention according to claim 3, wherein said working fluid is amaterial selected from the group consisting of potassium and lithium.

1. In combination: a tubular member having spaced inner and outer wallsand interconnecting end walls defining a closed, evacuated annularchamber surrounding a central opening; means forming a capillarystructure on the inner surface of said outer wall and a capillarystructure on the outer surface of said inner wall; additional capillarymeans interconnecting said capillary structures and including aplurality of groups of wick elements spacing said capillary structures,said wick elements being circumferentially and longitudinally spacedwithin said annular chamber to provide continuous fluid flow pathsthrough said chamber; and said annular chamber being evacuated of allnon-condensable gases and being partially filled with a substance whichis vaporizable from a liquid phase and is capable of transport throughsaid capillary structures and additional capillary means.
 2. A heat pipecomprising: a tubular member having spaced inner and outer walls andintersecting end walls forming a closed, evacuated chamber including anannular space between said spaced walls; a capillary structure on theinner surface of said outer wall and a capillary structure on the outersurface of said inner wall; additional capillary means interconnectingsaid capillary structures and including a plurality of groups of wickelements spacing said capillary structures, said wick elements beingcircumferentially and longitudinally spaced within said annular space toprovide continuous fluid flows paths within said chamber; and a workingfluid partially filling said chamber and formed of a substance that isvaporizable from a liquid phase and is capable of transport through saidcapillary structures and said additional capillary means.
 3. A heat pipecomprising: a pair of radially spaced inner and outer walls extendinglongitudinally and circumferentially and closed by end walls to form ahermetically sealed chamber including an annular space between saidradially spaced walls; a capillary structure on the inner surface ofsaid outer wall and on the outer surface of said inner wall; additionalcapillary means interconnecting said capillary structures and includinga plurality of wick elements extending longitudinally and radially andspaced longitudinally within said annular space to provide continuousfluid flow paths within said chamber; and a working fluid within saidchamber that is vaporizable from the liquid phase and capable oftransport through said capillary structures and additional capillarymeans.
 4. The invention according to claim 3, wherein said wallscomprise a pair of tubular members substantially equal and coextensivein length; said end walls joining the adjacent ends of said tubularmembers to form an elongated annular chamber surrounding an openpassageway.
 5. The invention according to claim 4, wherein said tubularmembers are circularly cylindrical.
 6. The invention according to claim3, wherein said capillary structures and said wick elements are formedof metal mesh.
 7. The invention according to claim 3, wherein saidworking fluid is a material selected from the group consisting ofpotassium and lithium.